1. Field of the Invention
This invention relates to an injection molding nozzle for use in conjunction with an injection molding machine, and especially (but not exclusively) in the context of a hot runner manifold. More specifically, the invention relates to a method and apparatus for controlling a thin heater with low thermal inertia and retaining it on, and in thermal contact with, a body, such as an injection molding nozzle.
2. Background Information
An injection molding apparatus may include a heated hot runner manifold for the distribution of a molten material to at least one injection nozzle. Each injection nozzle may be associated with one or more mold cavities, whereby the molten material is transferred to the mold cavity through a gate orifice located at a distal end of the nozzle. During an injection cycle, the gate orifice preferably may be selectively opened and closed to start and stop the flow of molten material to the respective mold cavity.
Typically, an injection molding nozzle will include a heater placed in contact with the exterior of the nozzle in many well known configurations. One such heater is a wire wound resistive heater that is sized to slip over the exterior diameter of the injection nozzle. To control the temperature of the molten material in the injection nozzle, a temperature controller is typically connected to the heater and a thermocouple placed in close proximity to the injection nozzle tip. Such thermocouples are typically installed in an aperture in the nozzle housing or tip retainer. An operator uses the temperature controller to select a temperature set point, and electrical power to the heater is varied by the temperature controller in accordance with the temperature set point and a signal from the thermocouple. It is well known in the prior art that the measured temperature as reported by the thermocouple can be significantly different from the actual temperature of the molten material in the nozzle. Operators typically must run trial and error experiments to determine the proper temperature set point for each new setup to reliably produce an injection molded part. These trial and error experiments can take considerable time and waste valuable resources.
Experiments have shown that in prior art nozzles, there exists a large thermal gradient along the length of the injection nozzle. Having a non-constant temperature along the length of the nozzle subjects the sometimes-sensitive resin to hot spots as it flows towards the mold cavity. These hot spots can degrade the resin and result in a low-quality molded part. Since plastics are often sensitive to temperature, degradation of the processed melt can also occur as a consequence of any errors in temperature measurement. In addition, these factors may cause the temperature operating window for a particular setup with a given resin to be very narrow. Experiments have shown that axial placement of the heater along the nozzle significantly impacts the thermal profile of the molten material in the nozzle, and position of the thermocouple relative to the heater dramatically affects the temperature reading it provides.
Ideally, it would be advantageous to provide a nozzle having constant temperature (isothermal) along its entire length, and in which a one degree difference in the set point temperature would result in a corresponding one degree difference in the nozzle temperature.
U.S. Pat. No. 5,360,333 to Schmidt and U.S. Pat. No. 5,411,392 to Von Buren (each incorporated herein by reference) both disclose means for clamping a wire-wound cartridge-type bi-metal heater to an injection nozzle using thermal expansion properties of the bimetallic heater to tightly clamp it against the nozzle when hot. However, these patents do not address axially positioning the heater relative to the nozzle, nor placement of a thermocouple relative to the heater or nozzle.
FIG. 7 illustrates how a heater disclosed in Schmidt is typically mounted on a nozzle assembly. In a cold condition, heater 202 can slide axially over nozzle subassembly 200 and is positioned as shown. A conventional retaining ring 204 is then installed in a circumferential groove 206 in nozzle subassembly 200 to prevent the heater 202 from moving toward tip 208. A thermocouple 210 is installed in an aperture 212 in heater 202, then cap 214 is screwed onto heater 202, with threads 216 engaging threads 218, to retain thermocouple 210 in aperture 212 and prevent heater 202 from moving axially. It should be noted that thermocouple 210 is placed adjacent one of the heat-producing elements 220 of heater 202 to provide a good indication of the temperature of the heater near the nozzle tip. Aperture 212 does not extend through heater 202 so that thermocouple does not touch the nozzle subassembly 200. This prevents damage to the thermocouple 210 when heater 202 with thermocouple 210 is removed from nozzle subassembly 200.
Recently, advances in heater technologies have produced film heaters that require much less space, less power and are more reliable than wire wound cartridge-type heaters. U.S. Pat. Nos. 5,973,296 to Juliano et al. 6,305,923 and U.S. Pat. No. 6,341,954 to Godwin et al. and U.S. patent application Ser. No. 09/596,549 (each incorporated herein by reference) disclose the current state of the art of film-based heater technology specifically adapted for use on injection molding machines and the like. However, due to the nature of the film resistive elements that have a very well-defined operating temperature window, the actual operating temperature of the film heater should be precisely controlled to avoid premature failure. Therefore, when measuring the temperature of a film heater, it is beneficial to place the thermocouple, or other temperature sensor, adjacent a heat producing element of the film heater. Because the substrate of film heaters is typically much thinner than that of the cartridge-type heaters, the technique for retaining a cartridge-type heater on the nozzle and mounting a thermocouple on it, as illustrated in FIG. 1, is not well suited to film heaters. If oriented similar to aperture 212, an aperture of sufficient length to properly engage a thermocouple in a film heater would go through the film heater substrate leaving the thermocouple end rubbing against the nozzle assembly, thereby making it susceptible to damage when the heater is removed. The thinner substrate of the film heater also makes threading it more problematic. A threaded connection is susceptible to becoming loose after numerous molding cycles, and if overtightened, it can also be difficult to unscrew after being heated. Accordingly, an improved apparatus for attaching a film heater to an injection molding nozzle, or the like, and measuring and controlling the temperature of the heater, and thus the nozzle, is required.